The instant invention relates to apparatus for providing an indication of compaction in vibration compacting machines of the type which comprise:
(a) a vibrating working part; PA1 (b) an acceleration pick-up mounted on the vibrating working part; PA1 (c) means for preselecting an acceleration value; PA1 (d) means for preselecting consecutive observation intervals; PA1 (e) means for responding to whether or not the preselected acceleration value is exceeded within each observation interval; and PA1 (f) control means for increasing or reducing the preselected acceleration value depending on whether such an acceleration occurs or does not occur.
Belgium Pat. No. 673 215 discloses an acceleration pick-up which is mounted on a vibrating roller. The output signal is processed to generate a signal corresponding to the amplitude of the vibrating roller. When this signal attains a certain magnitude, an indicating or control signal is triggered. The device disclosed in the Belgium patent was designed based on the assumption that the compacting effect of the soil compacting machine is optimum when the vibrating roller oscillates with the resonant frequency of the soil to be compacted because the oscillation amplitude of the roller is at its maximum and thus the effect on the soil to be compacted is maximized. This assumption is not always justified because it presumes that the vibrating roller or working part makes a harmonic movement.
The working part of many dynamic soil compacting machines does not make a harmonic movement but rather operates in a jumping fashion. The working part describes a substantially balistic trajectory under the action of gravity and possible pre-loading forces. The harmonic excitation forces of the vibration generator may not be in the direction of travel of the working part when it hits the soil. When the excitation forces at this moment are directed downwardly, they support the impact effect of the working part when hitting the soil. However, the impact effect is diminished when the excitation forces are at this moment directed upwardly. When this occurs, oscillation amplitude is not a true measure of the compaction obtained by the vibration compacting machine.
A number of forces act on the soil compacting machine: the impact force effective between the working part and the soil; the inertia force of the working part; and the excitation force and the pre-loading forces, which are in equilibrium. The maximum value of the impact force obtained when the working part hits the soil represents the value determining soil compaction. The maximum value of the impact force is a multiple of the excitation force and the pre-loading forces. An output signal corresponding to the impact force to be detected can be generated by an acceleration pick-up, the signal from which is proportional to the inertia of the working part.
German Pat. No. 20 66 015 discloses a device for controlling the compaction in vibration compacting machines in which an acceleration pick-up having a vertically moveable guided mass is mounted on the working part with the mass forming the armature of an electromagnet. The magnetic flux of the electromagnet is variable so that a certain retaining force of the electromagnet can be preselected, and thus also a certain acceleration value, at which the armature is torn from the magnet. When such occurs, an induction winding on the electromagnet generates a signal thereby indicating that the preselected acceleration value has been exceeded. Thus, the impact force at the moment the working part strikes the soil then exceeds a predetermined value thereby indicating that a specific degree of soil compaction has been exceeded. Although indicating the degree of compaction exceeded, no indication of the degree of compaction reached is provided.
In another known apparatus for providing an indication of the compaction in vibration compacting machines, preselected observation intervals are defined. Within each interval, it is observed whether a preselected impact force, which is determined by an acceleration pick-up mounted on the working part, is exceeded. If so, the preselected acceleration value is increased; if not, the preselected acceleration value is reduced. The acceleration pick-up is of the same type as described in German Pat. No. 20 66 015. Depending on whether the pick-up has responded, that is whether the predetermined acceleration value has been exceeded, a voltage of positive or negative sign is applied to an integrator, the output of which increases or reduces the current in a winding of the electromagnet via a voltage-to-current transducer. Increasing or decreasing the winding current proportionally increases or decreases the acceleration value at which the armature is torn from the electromagnet. If, within a certain observation interval, the acceleration value determined by the energizing current of the electromagnet is not exceeded, the compaction material therefore is not yet compacted to the degree of compaction corresponding to this acceleration value. Therefore, the energizing current of the electromagnet and thus the preselected acceleration value is reduced through the integrator. Such reduction occurs until the reduced acceleration value is exceeded within an observation interval. Such leads to an inversion of the input voltage of the integrator and to an increase of the preselected value. Thus, the energizing current of the electromagnet varies about a value which corresponds to the obtained degree of compaction. This energizing current provides an indication of the degree of the compaction obtained.
As previously mentioned, the impact forces occuring within each impact of the working part on the soil are dependent on different influences and are statistically distributed. The observation in consecutive observation intervals amounts to taking samples of the impact forces. Quasi-stationary jumping behavior of the working part occurs when compaction is complete. Under such circumstances the variation range of the energizing current, and thus of the predetermined acceleration values, corresponds to the muting interval for the threshold of a certain fractile limit, e.g., the fractile of ninety percent (90%) of the distribution of the relative maximums of the acceleration of the working part when compacting the compaction material. The increase or reduction of the energizing current in the acceleration pick-up depending on whether the predetermined acceleration value is exceeded or not within the observation interval represents simulation of a value of the impact acceleration of the working part which corresponds to a certain fractile limit of the values occuring in the sample.
The compaction indicated in this way is subject to uncertainty due to the periodic variation between upper and lower limits under conditions in which the soil is substantially completely compacted. An indicating instrument, e.g., a meter, monitoring the acceleration value so determined oscillates between upper and lower limits and it can only be determined that the acceleration value, and thus the actual degree of compaction, is located somewhere between these limits. It is possible to reduce the oscillation between these upper and lower limits by reducing the change of the energizing current of the electromagnet and thus the preselected acceleration value. Under such conditions, the indications for consecutive observation intervals vary slowly. When the indication increases, it can only be determined that an acceleration which exceeded the preselected acceleration value occured within the observation interval. An upper limit for the occuring acceleration is not established until after a plurality of observation intervals when the predetermined acceleration in question is no longer exceeded and reversal of movement of the indication takes place. Thus, variations of the degree of compaction of the soil are not indicated substantially as they occur but rather are delayed.
Such past apparatus therefore provide either a heavily oscillating indication, the value of the actual degree of compaction being located anywhere in the variation interval, or a slowly oscillating indication which has a lower level of information and which does not permit the detection of quick variations in the degree of compaction.